Printed transistors

ABSTRACT

A transistor is formed by applying modifier coatings to source and drain contacts and/or to the channel region between those contacts. The modifier coatings are selected to adjust the surface energy pattern in the source/drain/channel region such that semiconductor printing fluid is not drawn away from the channel region. For example, the modifier coatings for the contacts can be selected to have substantially the same surface energy as the modifier coating for the channel region. Semiconductor printing fluid deposited on the channel region therefore settles in place (due to the lack of a surface energy differential) and forms a relatively thick active semiconductor region between the contacts. Alternatively, the modifier coatings can be selected to have lower surface energies than the modifier coating in the channel region, which actually causes semiconductor printing fluid to be drawn towards the channel region.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to electronic materials processing, andmore particularly to a system and method for printing thin filmtransistor arrays.

2. Related Art

Many modern devices, such as video and computer LCD (liquid crystaldisplay) displays, include large arrays of thin film transistors (TFTs).These TFT arrays are commonly referred to as active matrix backplanesand are used to control the media in a display. As the demand for largerdevices rises, the corresponding increase in TFT array sizes andinterconnect complexities can render impractical the use of conventionalchamber-based semiconductor process techniques (i.e., processes that areperformed within a vacuum chamber).

Accordingly, alternative TFT production methods are taking on greaterimportance. A promising approach involves the printing of transistorsusing methods such as offset printing or jet printing. The use of suchintegrated circuit (IC) printing techniques can substantially lowertransistor production costs, as well as increase manufacturingflexibility, as the substrate material and environmental limitationsassociated with chamber-based processing techniques can be eliminated.

For example, FIGS. 1A, 1B, and 1C depict a cross-sectional view ofstages in a conventional IC printing operation. In FIG. 1A, a gate 130formed on a substrate 110 is covered by a dielectric 120. A sourcecontact 140 and a drain contact 150 on dielectric 120 define a channelregion 101 above gate 130.

In FIG. 1B, a semiconductor printing fluid 160′ is deposited intochannel region 101, onto a surface 120-S of dielectric 120 and on tosurfaces 140-S and 150-S of source contact 140 and drain contact 150,respectively. Semiconductor printing fluids are printable fluids thatdry to leave a semiconductor material. Thus, printing fluid portion 160′dries into a semiconductor region 160 that forms the active region for aTFT 100, as shown in FIG. 1C.

Unfortunately, as indicated in FIG. 1C, the semiconductor region 160 inconventional printed TFT 100 typically exhibits a very “spread out”geometry, in which only a non-uniform layer of semiconductor materialremains in channel region 101. These non-uniformities may comprisevariations in thickness of, holes in, or discontinuities in thesemiconducting layer. This in turn can lead to poor transistorperformance or even device failure, if a discontinuity 160-B develops insemiconductor region 160.

This spreading of printing fluid portion 160′ as it dries is due to thefact that dielectric 120 typically has a very different surface energyfrom source contact 140 and drain contact 150. Source contact 140 anddrain contact 150 are typically formed from some type of metal, whichmakes them inherently wettable (high surface energy). However, to ensureproper crystalline structure within semiconductor region 160 for someprintable semiconductors, it is typically required that surface 120-S ofdielectric 120 (on which printing fluid portion 160′ is deposited) benonwettable (low surface energy).

This disparity in surface energies leads to the problematic spreadgeometry of semiconductor region 160 in FIG. 1C, as the relativelywettable surfaces 140-S and 150-S of contacts 140 and 150, respectively,tend to draw printing fluid portion 160′ away from the relativelynonwettable surface 120-S in channel region 101. Conventional techniquesfor preventing this spread of semiconductor printing fluid 160′ havetypically involved constructing physical barriers at contacts 140 and150 or by printing a large amount of excess material (i.e., depositingmuch more semiconductor printing fluid 160′ than is necessary in anattempt to ensure that semiconductor region 160 is continuous in channelregion 101). However, these approaches can add a great deal of cost andcomplexity to the printed transistor manufacturing process, and aretherefore generally undesirable solutions.

Accordingly, it is desirable to provide a system and method for printinghigh-quality transistors that does not require the formation of physicalcontainment structures on the transistors.

SUMMARY OF THE INVENTION

The invention is directed towards IC printing system and techniques thatuse semiconductor printing fluids to create semiconductor structures. Byapplying modifier coatings to the source/drain contacts and/or thechannel region of a transistor, a semiconductor printing area is createdthat is conducive to the formation of relatively uniform, continuousactive semiconductor regions in printed transistors.

A modifier coating is formed by applying a modifier fluid to a surface.The modifier molecules in the modifier fluid are selected to have anaffinity for particular materials, and can be used to selectively coattransistor structures. A contact modifier coating can therefore beapplied to just the source/drain contacts, while a channel modifiercoating can be applied to the channel region between the contacts.

According to an embodiment of the invention, the contact and channelmodifiers can be selected to have substantially similar surfaceenergies. Therefore, a portion of semiconductor printing fluid dispensedonto the channel region will settle in place, rather than spread outaway from the channel region as in conventional printed transistors. Theresulting semiconductor region formed as the semiconductor printingfluid dries provides a continuous, relatively uniform active region thatensures proper transistor operation.

According to another embodiment of the invention, the contact modifiercan be selected to have surface energy that is lower than the surfaceenergy of the channel modifier. Therefore, a portion of semiconductorprinting fluid dispensed onto the channel region will be drawn towardsthe channel region from the contact regions, thereby providing a degreeof self-alignment for the semiconductor printing operation.

According to various other embodiments of the invention, modifiercoatings can be applied to either the source/drain contacts or to thechannel region. In either case, the modifiers can be selected to ensurea surface energy pattern between the contacts and channel region thatensures proper active region formation from semiconductor printing fluiddispensed onto the channel region.

The selective nature of the modifiers allows the modifier coatings to becreated using a low-cost dip or spray process. According to anembodiment of the invention, a roll-processing system includes a coatingmodule (system) for applying modifier coatings to the source/draincontacts and/or channel regions of circuits printed on a flexiblesubstrate, in preparation for printing of semiconductor active regions.The ability to perform the coating operations using (non-chamber-based)dip or spray operations enables the entire IC printing operation to beon the flexible substrate straight from the roll (i.e., continuousprocessing).

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawings, where:

FIGS. 1A, 1B, and 1C, are cross-sectional views of stages in aconventional TFT printing operation;

FIGS. 2A, 2B, 2C, 2D, and 2E are cross-sectional views of stages in aTFT printing operation in accordance with an embodiment of theinvention;

FIGS. 2F, 2G, and 2H are cross-sectional views of printed TFT inaccordance with various other embodiments of the invention;

FIG. 2I is a cross-sectional view of a printed TFT array in accordancewith an embodiment of the invention.

FIG. 3 is a top view of a printed TFT in accordance with an embodimentof the invention;

FIGS. 4A, 4B, 4C, 4D, 4E, 4F, and 4G are cross-sectional views of stagesin a TFT printing operation in accordance with another embodiment of theinvention;

FIG. 5 is a flow diagram of an IC printing operation in accordance withan embodiment of the invention; and

FIG. 6 is a diagram of an IC printing system in accordance with anembodiment of the invention.

DETAILED DESCRIPTION

By adjusting the surface characteristics of transistor elements, thequality and accuracy of printed semiconductor elements can be improved,thereby reducing the cost and improving the performance of printedtransistors. For example, FIGS. 2A, 2B, 2C, 2D, and 2E depictcross-sectional views of a transistor formation process in accordancewith an embodiment of the invention.

FIG. 2A depicts an intermediate stage in the formation of a printedtransistor. A gate 230 is formed on a substrate 210, and is covered by adielectric 220. A source contact 240 and a drain contact 250 (i.e., acontact pair) are formed on dielectric 220 and define a channel region201 above gate 230. Gate 230, dielectric 220, and contacts 240 and 250can be formed using any method, including conventional chamber-basedprocessing techniques or IC printing techniques. Similarly, substrate210 can comprise any substrate material, including a silicon wafer or aflexible plastic film.

As described above, surfaces 240-S and 250-S of contacts 240 and 250,respectively, will typically exhibit a surface energy that is differentthan the surface energy of a surface 220-S of dielectric 220. Forexample, contacts 240 and 250 will typically be a metal such asaluminum, copper, or gold (or sometimes even doped polysilicon) thatexhibits wettable surfaces 240-S and 250-S, respectively, whiledielectric 220 will typically be an inorganic oxide or insulatingpolymer having a lower surface energy than that of the contacts 240 and250.

As described above with respect to FIGS. 1A–1C, this typical surfaceenergy distribution across the source/drain contacts and channel regionof a TFT is far from ideal for subsequent semiconductor printingoperations. According to various embodiments of the invention, one ormore modifier coatings (described in greater detail below with respectto FIGS. 2B and 2C) can be used to overcome this disadvantageous surfaceenergy pattern.

A modifier contains a chemical moiety (component) that is reactivetowards the surface being coated. By using modifiers that are onlyreactive towards the surfaces for which coverage is desired, selectivecoverage of a heterogeneous surface can be readily achieved. Forexample, by applying a modifier fluid (i.e., a fluid containing modifiermolecules or compounds) that is reactive towards dielectric 220 butnon-reactive towards contacts 240 and 250 (e.g., an alkyltrichlorosilane or alkyl trimethoxysilane if dielectric 220 is a siliconoxide and contacts 240 and 250 are metals), modifier coating 221 can beformed over only surface 220-S of dielectric 220 using a simple dip orspray process. Note however, that the formation modifier coating 221(and any other modifier coating described herein) can be formed using avariety of different techniques, including vapor deposition or evendirect printing.

According to an embodiment of the invention, modifier coating 221 cancomprise a monolayer of molecules, sometimes referred to as aself-assembled monolayer (SAM). The SAM could be formed by dissolvingthe desired modifier molecule (e.g., alkyl trichlorosilane) in a solvent(e.g., hexadecane or toluene), and then dipping substrate 210 (and theoverlying structures) into the solvent (e.g., in a bath or spray). Themodifier molecules then assemble onto exposed surfaces of dielectric 220to form modifier coating 221. Because the modifier molecules arenon-reactive towards contacts 240 and 25Q, no coating is formed on thosestructures.

Similarly, modifier coatings 241 and 251 can be formed on surfaces 240-Sand 250-S, respectively, of contacts 240 and 250, respectively, as shownin FIG. 2C. Once again, by selecting a modifier material that isreactive towards the material of contacts 240 and 250 (e.g., anorganothiol if contacts 240 and 250 are coinage metals, such aspalladium or gold), but non-reactive towards modifier coating 221,surfaces 240-S and 250-S of contacts 240 and 250 can be selectivelycoated. In this manner, a receiving surface 202 (the extent of which isindicated by the dotted line) is created on which semiconductor printingfluid can subsequently be deposited.

Note that because of the selective nature of modifier coatings 221, 241,and 251, the order in which those coatings are formed can be varied. Forexample, according to another embodiment of the invention, modifiercoatings 241 and 251 can be formed over surfaces 240-S and 250-S,respectively, of contacts 240 and 250, respectively, prior to theformation of modifier coating 221 over surface 220-S of dielectric 220.Note also that since dielectric 220 is generally formed from a differentmaterial than contacts 240 and 250, modifier coating 221 will generallyalso be different from modifier coatings 241 and 251 (which themselvesmay be different if contacts 240 and 250 are formed from differentmaterials).

According to an embodiment of the invention, modifier coatings 241 and251 can be monolayers, thereby minimizing the effect of coatings 241 and251 on the electrical properties of contacts 240 and 250, respectively.Because monolayers are only a single molecule thick, electrical signalscan pass through them without significant degradation or attenuation.

The use of modifier coatings 221, 241, and 251 allows the surfacecharacteristics of receiving surface 202 at channel region 201 and oversource contact 240 and drain contact 250 to be set to desired values.Specifically, the surface energy of receiving surface 202 at eachlocation can be adjusted to provide a desired surface energy pattern forthe printing of a semiconductor region.

For example, according to an embodiment of the invention, modifiercoatings 221, 241, and 251 could be selected to have substantially thesame surface energy characteristics. Then, a semiconductor printingfluid 260′ deposited onto receiving surface 202 as shown in FIG. 2D (forexample, via jet printing) would spread out mainly in response to itsown surface tension. Note that while semiconductor printing fluid 260′is depicted as extending from contact 240 to contact 250 (across channelregion 201) for exemplary purposes, according to various otherembodiments of the invention, semiconductor printing fluid 260′ may bedeposited onto receiving surface 202 over only channel region 201 orover channel region 201 and one of contacts 240 and 250 (withsemiconductor printing fluid 260′ extending to both contacts 240 and 250during the drying process).

Semiconductor printing fluid 260′ can comprise any semiconductormaterial in a carrier liquid, and can be a suspension, dispersion,solution, or any other liquid form. Examples of semiconductor printingfluids include, but are not limited to, semiconducting polymers such aspoly(3-hexylthiophene), P3HT, orpoly[5,5′-bis(3-dodecyl-2-thienyl)-2,2′-bithiophene], PQT-12 dissolvedin an organic solvent (e.g. chlorobenzene). Dispersions of inorganicsemiconducting nanoparticles or colloids are another example of asemiconductor printing fluid. Because the dissimilar surface energies ofdielectric 220 and contacts 240 and 250 are “hidden” by modifiercoatings 221, 241, and 251, printing fluid 260′ is not pulled away fromchannel region 201.

Thus, as shown in FIG. 2E, printing fluid 260′ is able to settle inplace (in response to any internal surface tension) and dry into asemiconductor active region 260 that provides a continuous layer ofsemiconductor material that is relatively uniform across channel region201 (i.e., semiconductor active region 260 is at least as thick orthicker over channel region 201 than it is over contacts 240 and 250).This allows the final TFT 200 to provide reliable transistor operation.Note that active semiconductor region 260 also spreads out over portions242 and 252 of modifier coatings 242 and 252, respectively, therebyensuring good electrical contact with contacts 240 and 250,respectively. In some embodiments, the active semiconductor region 260may only be in contact with the edge of modifier coatings 242 and 252 oncontacts 240 and 250 respectively.

According to another embodiment of the invention, modifier coatings 241and 251 formed on contacts 240 and 250, respectively, could be selectedto have lower surface energies than modifier coating 221 formed overdielectric 220. As a result, the portions of receiving surface 202 overcontacts 240 and 250 are more nonwettable than the portion of receivingsurface 202 in channel region 201.

Therefore, a semiconductor printing fluid 260′ deposited over channelregion 201 and contacts 240 and 250 tends to settle and dry mainly overchannel region 201, forming a well-defined, relatively uniform activeregion 260-2 for a TFT 200-2, as shown in FIG. 2F. Note that asemiconductor region 260-2 formed in this manner will tend to coversmaller portions 242-2 and 252-2 of modifier coatings 241 and 251,respectively, than a semiconductor region (260 in FIG. 2E) formed over arelatively homogeneous surface energy distribution. However, transistor200-2 will generally function properly with very small active regioninterfaces (242-2 and 252-2), since even those small active regioninterfaces can provide the necessary electrical contact with contacts240 and 250, so long as a substantial active semiconductor region 260(i.e., providing sufficient charge carriers) is provided across channelregion 201. The act of depositing the semiconductor printing fluid(260′) onto channel region 201 can automatically create some contactbetween semiconductor region 260 and contacts 240 and 250, if theminimum drop size of IC printing systems is larger than the desiredchannel length.

In this manner, modifier coatings 221, 241, and 251 can be used toprovide self-alignment capability for semiconductor printing operations.Note that while forming modifier coatings over both dielectric 220 andcontacts 240 and 250 provides the greatest flexibility in defining asurface energy pattern, according to various other embodiments of theinvention, modifier coatings can be formed over just contacts 240 and250, or over just dielectric 220.

For example, FIG. 2G shows a TFT 200-3 that is substantially similar toTFT 200 shown in FIG. 2E, except that a modifier coating has not beenformed over dielectric 220. By selecting modifier coatings 241 and 251to be as nonwettable as dielectric 220, a receiving surface formed bymodifier coating 241, the portion of dielectric 220 in channel region201, and modifier coating 251 provides a surface energy pattern thatallows a semiconductor active region 260-1 to be printed that provides arelatively thick semiconductor layer across channel region 201 (forreasons substantially similar to those described with respect to FIG.2E). According to another embodiment of the invention, modifier coatings241 and 251 could be selected to be more nonwettable than dielectric220, thereby creating a receiving surface that provides theself-aligning aspect for semiconductor active region 260-1 describedwith respect to FIG. 2F.

Alternatively, FIG. 2H shows a TFT 200-4 that is substantially similarto TFT 200 shown in FIG. 2E, except that modifier coatings have not beenformed over contacts 240 and 250. By selecting modifier coating 221 tobe as wettable as contacts 240 and 250, a receiving surface formed bysource 240, the portion of modifier coating 221 in channel region 201,and drain 250 provides a surface energy pattern that allows asemiconductor active region 260-2 to be printed that provides arelatively uniform, continuous semiconductor layer across channel region201 (for reasons substantially similar to those described with respectto FIG. 2E). According to another embodiment of the invention, modifiercoating 221 could be selected to be more wettable than contacts 240 and250, thereby creating a receiving surface that provides theself-aligning aspect for semiconductor active region 260-1 describedwith respect to FIG. 2F (note that the material used to formsemiconductor active region 260-1 would then need to be selected toprovide proper crystalline structure when deposited on a wettablemodifier coating 221).

According to another embodiment of the invention, printed transistorswith modifier coatings can be used to form a transistor array. FIG. 2Ishows a cross-sectional view of a transistor array A200 (e.g., a TFTarray suitable for a display backplane) that includes transistors 200-1,200-2, and 200-3, each of which is substantially similar to transistor200 shown in FIG. 2E. Note that a transistor array in accordance withthe invention can include any number and arrangement of transistors.Note further that while transistors 200-1, 200-2, and 200-3 are depictedas being substantially similar to transistor 200 for exemplary purposes,according to various other embodiments of the invention, transistors200-1, 200-2, and 200-3 can comprise any transistor structure thatincludes modifier coatings for surface energy patterning (e.g.,transistors 200-2, 200-3, and 200-4 shown in FIGS. 2F, 2G, and 2H,respectively).

FIG. 3 is a micrograph of a top view of a TFT 300 in accordance with anembodiment of the invention. TFT 300 is substantially similar to TFT200-1 shown in FIG. 2F, and includes a coated dielectric layer 320formed over a gate 330, a coated source contact 340 and a coated draincontact 350 formed on coated dielectric layer 320, and a semiconductoractive region 360 formed over gate 330 between contacts 340 and 350.Note that for exemplary purposes, the micrograph of FIG. 3 shows all theTFT structures, including gate 330, which is actually covered by coateddielectric layer 320.

Coated dielectric layer 320 includes a dielectric material and a firstmodifier coating (as described with respect to FIG. 2B), while contacts340 and 350 are metal contacts coated with a second (different) modifiercoating (as described with respect to FIG. 2C). Note that coated sourcecontact 340 and coated drain contact 350 are actually formed on uncoatedportions of coated dielectric layer 320, since contacts 340 and 350 areformed before dielectric layer 320 is coated.

The modifier coating of coated source contact 340 and coated draincontact 350 is selected to be more nonwettable than the modifier coatingof coated dielectric layer 320. Therefore, a drop of semiconductorprinting fluid deposited over gate 330 and contacts 340 and 350 willtend to flow towards the channel region between contacts 340 and 350(for reasons described with respect to FIG. 2F), thereby causingsemiconductor active region 360 to be aligned with gate 330.

Note that while a particular TFT structure is described in FIGS. 2A–2Hfor exemplary purposes, the use of modifiers to adjust surface energypatterns for IC printing operations can be applied to any transistorstructure where control over printed semiconductor elements isimportant. For example, FIGS. 4A, 4B, 4C, 4D, 4E, 4F, and 4G depictcross-sectional views of a transistor formation process in accordancewith another embodiment of the invention.

FIG. 4A shows a cross-section of a source contact 440 and a draincontact 450 formed on a substrate 410, with source contact 440 and draincontact 450 defining a channel region 401. A modifier coating 411 isformed over a surface 410-S of substrate 410 in FIG. 4B. Modifiercoating 411 is selected to be reactive only with substrate 410, so thatsurfaces 440-S and 450-S of contacts 440 and 450, respectively, are leftuncoated. According to an embodiment of the invention, modifier coating411 is selected to be relatively nonwettable towards the semiconductorprinting fluid (shown in FIG. 4D) to ensure high-quality printedsemiconductor formation.

Next, in FIG. 4C, modifier coatings 441 and 451 are formed over surfaces440-S and 450-S, respectively, of contacts 440 and 450, respectively.Modifier coatings 441 and 451 (which can be formed using the samemodifier fluid) are selected to be reactive only with contacts 440 and450, and so can be applied using a dip or spray process. Just asdescribed with respect to FIGS. 2B and 2C, the selective nature ofmodifier coatings 411, 441, and 451 allows those coatings to be formedin any order. Note also that according to an embodiment of theinvention, modifier coatings 441 and 451 can comprise SAMs to minimizethe electrical effects of modifier coatings 441 and 451. In this manner,a receiving surface 402 (the extent of which is indicated by the dottedline) is created on which semiconductor printing fluid can subsequentlybe deposited.

According to various embodiments of the invention, modifier coatings411, 441, and 451 can also be selected to have substantially similarsurface energies, or can be selected such that modifier coating 411 hasa greater surface energy than coatings 441 and 451 (i.e., modifiercoatings 441 and 451 are more nonwettable than modifier coating 411).Thus, a semiconductor printing fluid 460′ deposited over receivingsurface 402 as shown in FIG. 4D, will either settle in place (homogenoussurface energy pattern) or will be drawn substantially into channelregion 401 (modifier coatings 441 and 451 being more nonwettable thanmodifier coating 411). In either case, a relatively thick activesemiconductor region 460 is formed over channel region 401, as shown inFIG. 4E.

Then, in FIG. 4F, a dielectric 420 is formed over the structure shown inFIG. 4E. Finally, a gate contact 430 is formed on dielectric 420 overchannel region 401 to complete TFT 400, as shown in FIG. 4G. In thismanner, a high-quality transistor can be produced using a semiconductorprinting fluid. Note that according to an embodiment of the invention,source contact 440, drain contact 450, and gate contact 430 can all beformed using IC printing techniques, thereby allowing TFT 400 to beproduced without any chamber-based process steps.

FIG. 5 shows a flow diagram for the transistor formation processesdescribed with respect to FIGS. 2A–2H and 4A–4G, according to anembodiment of the invention. If a transistor similar to TFT 200 shown inFIG. 2E is being formed, in an optional “GATE/DIELECTRIC FORMATION”block 510, a dielectric layer (220) is formed over a gate contact (230)on a substrate (210). A source contact (240) and a drain contact (250)are then formed on the dielectric layer to define a channel region (201)over the gate contact in a “CREATE SOURCE/DRAIN CONTACTS” block 520.According to an embodiment of the invention, the source, drain, and gatecontacts can be formed using IC printing techniques.

Alternatively, if a transistor similar to TFT 400 shown in FIG. 4G isbeing formed, optional block 510 is not executed, and in block 520, asource contact (440) and a drain contact (450) are formed on a substrate(410) to define a channel region (401). Once again, according to anembodiment of the invention, the source and drain contacts can be formedusing IC printing techniques.

In either case (i.e., block 510 executed or not executed), after thecontact formation in block 520, the channel region and/or the contactsare coated with modifiers selected to provide a desired surface energypattern for subsequent semiconductor printing fluid deposition.According to an embodiment of the invention, a first modifier coating(221 or 411) is applied to the channel region (and other exposedportions of the dielectric or substrate) in a “CHANNEL REGION TREATMENT”block 530, and a second modifier coating (241/251 or 441/451) is appliedto the source and drain contacts in a “SOUCE/DRAIN TREATMENT” block 540.

As described above, because of the selective nature of modifiercoatings, blocks 530 and 540 can be performed in any order (or evensimultaneously, if modifiers having the appropriate reactivities areselected). According to an embodiment of the invention, the modifiercoatings formed over the source and drain contacts in block 540 can bemonolayers to minimize the effect of the modifier coatings on electricalconnectivity.

The channel region and source/drain contact modifier coatings areselected to provide a surface energy pattern that is conducive to properactive region formation, as described above with respect to FIGS. 2C–2Fand FIGS. 4C–4E. Thus, when a semiconductor printing fluid (260′ or460′) is deposited over the channel region in an “ACTIVE REGIONPRINTING” block 550, the fluid is not drawn away from the channel regionand can dry to provide a relatively thick, continuous semiconductorstructure (260 or 460) at the channel region.

At this point, the process for creating a transistor similar to TFT 200shown in FIG. 2E would be substantially complete. However, if atransistor similar to TFT 400 shown in FIG. 4G is being formed, anoptional “DIELECTRIC/GATE FORMATION” block 560 is executed to cover theactive semiconductor structure with a dielectric layer and form a gatecontact (430) over the channel region.

Note that according to another embodiment of the invention, either block530 or block 540 could be skipped, so that only the source/draincontacts or the layer on which the source/drain contacts are formed(e.g., the dielectric layer or the substrate) receive a modifier coating(as described with respect to FIGS. 2G and 2H). Note further that sincenone of the blocks in the flow diagram of FIG. 5 require chamber-basedprocessing, the method of the flow diagram can be used in continuousroll-type processing systems.

For example, FIG. 6 shows a schematic diagram of a continuousroll-processing system 600, in accordance with an embodiment of theinvention. System 600 includes a spool 610 on which is wound a roll offlexible substrate 620, a pre-coating module 630, a coating module 640,a post-coating module 650, a separator 660, and a transport system 670.Examples of flexible substrates include, but are not limited to, thinglass sheets, mylar, polyimide, or polyethylene naphthalate (PEN).

During operation of system 600, flexible substrate 620 is unwound fromspool 610 into pre-coating module 630, where contact pairs (i.e.,source/drain contacts) and any underlying transistor structures arecreated on flexible substrate 620. For example, as shown in FIG. 2A, agate contact (230) and dielectric layer (220) could be formed under thesource/drain contacts (240 and 250). Alternatively, as shown in FIG. 4A,the source/drain contacts (440 and 450) could be formed directly onflexible substrate 620. Note that pre-coating module 630 (as well ascoating module 640 and post-coating module 650) can include any numberof sub-modules to perform a desired set of processing operations).

Flexible substrate 620 continues into coating module 640, where one ormore modifier coatings are applied. According to an embodiment of theinvention, coating module 640 can include two submodules 641 and 642 forapplying different modifier coatings to the source/drain contacts and tothe layer on which the source/drain contacts are formed (e.g.,dielectric layer or substrate), as described with respect to FIGS. 2B–2Cand FIGS. 4B–4C. According to another embodiment of the invention,coating module 640 can apply a modifier coating to either source/draincontacts or the underlying layer, as described with respect to FIGS. 2Gand 2H.

As described above, the selective nature of the modifier coating(s)allows coating module 640 to be implemented as a dip or spray apparatus,which is well-suited for use in continuous roll-processing system 600.For example, as depicted in the cutaway of coating submodule 641, amodifier fluid 641-M could simply be sprayed onto flexible substrate 620to form a modifier coating on the structures on flexible substrate 620that are reactive with modifier fluid 641-M. According to an embodimentof the invention, coating module 640 can create SAMs over desiredstructures on flexible substrate 620.

Once the modifier coating(s) are formed, flexible substrate 620 travelsthrough a post-coating module 650, where a semiconductor region isprinted, as described with respect to FIGS. 2D–2E or FIGS. 4D–4E. Anyother required transistor or interconnect elements are also formed bypost-coating module 650 (e.g., dielectric 420 and gate contact 430described with respect to FIGS. 4F and 4G) to form completed printed ICson flexible substrate 620.

Flexible substrate 620 is then fed into separator 660, which cutsflexible substrate 620 into discrete flexible circuits 680, which canthen be transported to test, packaging, and/or any other destination bytransport system 670. Flexible circuits 680 can comprise any type ofcircuit that can be produced using the IC printing technique describedwith respect to FIG. 5. For example, according to an embodiment of theinvention, flexible circuits 680 could comprise large TFT arrays forminga display. Such displays could be produced relatively inexpensively bythe (non-chamber-based) continuous roll-process as electronic newspapersor magazines.

Although the present invention has been described in connection withseveral embodiments, it is understood that this invention is not limitedto the embodiments disclosed, but is capable of various modificationsthat would be apparent to one of ordinary skill in the art. Therefore,the invention is limited only by the following claims.

1. A first transistor comprising: a source contact formed on a baselayer; a drain contact formed on the base layer, the source contact andthe drain contact defining a channel region therebetween; at least oneof a first modifier coating on the source contact, a second modifiercoating on the drain contact, and a third modifier coating covering thebase layer in the channel region; and a semiconductor region extendingover the channel region and making electrical contact with the sourcecontact and the drain contact, wherein the semiconductor region contactsa receiving surface formed by one of the first modifier coating and thesource contact, one of the second modifier coating and the draincontact, and one of the third modifier coating and the base layer, andwherein at least one of the first modifier coating, the second modifiercoating, and the third modifier coating are selected such that a surfaceenergy of a first portion of the receiving surface in the channel regionis greater than or substantially equal to surface energies of a secondportion and a third portion of the receiving surface over the sourcecontact and the drain contact, respectively.
 2. The first transistor ofclaim 1, wherein the receiving surface includes the first modifiercoating, the second modifier coating, and the third modifier coating,and wherein the first modifier coating, the second modifier coating, andthe third modifier coating all have substantially the same surfaceenergy.
 3. The first transistor of claim 1, wherein the receivingsurface includes the first modifier coating, the second modifiercoating, and the third modifier coating, wherein the first modifiercoating has a first surface energy, wherein the second modifier coatinghas the first surface energy, and wherein the third modifier coating hasa second surface energy, the second surface energy being greater thanthe first surface energy.
 4. The first transistor of claim 1, whereinthe semiconductor region is formed from a semiconductor printing fluid,and wherein the receiving surface includes the third modifier coating,the third modifier coating being relatively nonwettable towards thesemiconductor printing fluid.
 5. The first transistor of claim 1,wherein the source contact and the drain contact comprise a metal,wherein the receiving surface includes the first modifier coating andthe second modifier coating, and wherein the first modifier coating andthe second modifier coating comprise an organothiol material.
 6. Thefirst transistor of claim 5, wherein the base layer comprises a silicondioxide layer, wherein the receiving surface includes the third modifiercoating, and wherein the third modifier coating comprises atrichlorosilane material.
 7. The first transistor of claim 1, whereinthe receiving surface includes the first modifier coating and the secondmodifier coating, wherein the first modifier coating comprises a firstmonolayer, and wherein the second modifier coating comprises a secondmonolayer.
 8. The first transistor of claim 1, wherein the base layercomprises a dielectric layer formed over a gate contact, the gatecontact being located under the channel region.
 9. The first transistorof claim 1, further comprising: a dielectric layer formed over thesemiconductor region and portions of the receiving surface not coveredby the semiconductor region; and a gate contact formed on the dielectriclayer over the channel region.
 10. The first transistor of claim 1,wherein the first transistor is included in a transistor array, thetransistor array comprising a plurality of transistors, the plurality oftransistors including the first transistor, each of the plurality oftransistors comprising the elements provided for the first transistor.